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My problem is that the elevation data does not perfectly match the satellite images. For example, on some small coastal zones, the sea actually reaches the top of some cliffs...

So I'was desperately looking for a way to manually correct the GDEM Aster elevation data (geoTIFF format files) for these zones. I tried using different photo editing software (trial versions and free versions) but none of them lets you set signed or unsigned 16 bit values to individual cells. Even though they are perfectly capable of processing 16 bit greyscale images, they only allow you to set 8 bit values (0-255) in their respective color pickers... I tried : Photoshop CS5, Photoline,, Picture window Pro Paint shop pro 10, Gimp 2

So I came across a software called Wilbur (version 1., which is very close to what I need.
Since It does apparently not accept geotiff files (.tiff format), I first converted my file to .png with photoshop and loaded it into Wilbur.
I corrected my map with Wilbur and then saved the file back to a .png surface (with another name of course).
When I got back to photoshop to revert it back to 16 bit Tiff, I noticed that the histogram had been compressed to the low and high values of the terrain, thus destroying the original data... In the file there is an island, whose original max height was 320 meters. After being processed with wilbur (where I only flattened false islands in the sea), its height culminated at over 16000 meters !!